Uncovering Electron-Transfer Mechanisms of Sulfur Anion Photosensitizers with Intramolecular Charge Transfer

Sulfur anions have emerged as promising photosensitizers for light-driven single-electron transfer (SET) chemistry. Established mechanistic pictures typically invoke substrate preassociation, either through electron donor-acceptor (EDA) complexation or via covalent adduct formation. Recent reports, however, describe sulfur-anion systems that operate efficiently in the apparent absence of these canonical pathways, highlighting an unresolved role for the intrinsic excited-state properties of sulfur anions. Here, we map the photochemical cycle of a representative sulfur-anion, PC3-S^-, photosensitizer by combining optical and magnetic spectroscopies. We show that its absorption originates from intramolecular charge-transfer (ICT) transitions and can be shifted into the visible region through π-system extension. Time-resolved measurements indicate that static and dynamic electron-transfer pathways can coexist, with the dominant route depending on substrate identity. In particular, polycyclic aromatic substrates favor static quenching, correlating with markedly faster reactions. In the investigation of light-induced reductive dehalogenation functionalization reactions, PC3-S^- demonstrated higher catalytic efficiency, in which TON up to 248 with 0.4 mol % loading. The data are consistent with non-covalent pre-association that leaves the sensitizer's intrinsic energy-level landscape largely unchanged, raising the possibility of anti-Kasha reactivity. Together, these findings provide spectroscopic constraints on bimolecular charge-transfer mechanisms of sulfur-anion photosensitizers and offer design principles for more efficient photocatalytic SET processes.